1. Field of the Invention
The present invention relates to an image sensing device, an image reading device, an image forming apparatus and an image sensing method.
2. Description of the Related Art
Charge-coupled device (CCD) image sensors have been used as a photoelectric conversion device for a scanner that reads an image in many cases. However, because of an increasing demand for low power consumption, complementary metal-oxide-semiconductor (CMOS) linear sensors are gaining more attention. A CMOS linear sensor converts incident light into electric charge by using photodiodes as in the case of a CCD image sensor. The CCD image sensor converts electric charge transferred by a shift register into voltage at a charge detector. The CMOS linear sensor converts electric charge into voltage signals at charge detectors provided for respective pixels and outputs the voltage signals via switches. By this configuration, the CMOS linear sensor consumes less power than the CCD image sensor.
The conventional CMOS linear sensors have a long analog bus connecting all the pixels to transfer analog image signals through the analog bus. Thus, wire resistance and wire capacitance are so large that the conventional CMOS linear sensors cannot achieve enhanced speed.
Japanese Patent Application Laid-open No. 2009-296544 describes sensors that scan three divided blocks, and each block is scanned three times to scan a whole line. The sensors simultaneously scan the divided blocks or a first block, a second block, and a third block in different colors in the first scan, the second scan, and the third scan.
However, the conventional CMOS linear sensor cannot achieve enhanced speed because of the difficulty in reducing wire resistance or wire capacitance along the analog bus.
Therefore, it is desirable to provide an image sensing device, an image reading device, an image forming apparatus, and an image sensing method that can achieve high-speed image reading.